1. Field of the Invention
The present invention relates to an information recording medium, a method of recording information thereon, and equipment thereof.
2. Description of the Prior Art
Recently, due to increasing information recording capacity of an optical recording medium, increasing distribution of information which is recorded on an optical recording medium and improving protection techniques against illegal copies, a technique for recording identification data on all optical recording media is desired for security purposes.
To service this demand, as recording identification data on an optical recording medium for example, the following technique is generally used: preparing a write-once type area (Burst Cutting Area, hereinafter referred to as a “BCA”) on which a barcode is overwritten on a pit of a read-only optical recording medium, and then during manufacturing of the optical recording medium, recording identification data (ID data) which is different in each optical recording medium and recording an encryption key or a reproducing key if necessary, on the BCA area.
On the other hand, a write-once optical information recording medium having an information layer which can record a signal only once and also including a plurality of recording layers, and a rewritable optical information recording medium having an information layer which can rewrite an information signal a number of times have been developed. Thus, various kinds of media have appeared (hereinafter, referred to as a “single-layer optical disk” including both the write-once type and the rewritable type). Furthermore, an optical disk having a plurality of recording layers which enables the recording of high-capacity information such as Hi-Vision images has been put into practical use (hereinafter, referred to as a “multiple-layer optical disk” including both the write-once type and the rewritable type).
In the single-layer or the multiple-layer optical disk, because information can be recorded without any particular restrictions, the emphasis on information handling for security purposes has increased.
As a method for recording ID data on an optical disk, an example of making the BCA by producing an amorphous portion and a crystalline portion alternatively by applying a laser beam onto a phase-changeable optical disk and using the difference of a reflecting ratio is proposed (see for example, Japanese unexamined patent publication 2001-243636, p. 16, FIG. 3, hereinafter referred to as reference 1).
However, in the conventional structure presented above, the identification data of a medium is recorded by applying a laser beam from the side at which information is recorded and reproduced. Therefore, the conditions for recording the identification data on a recording layer farthest from the laser incident side in a multiple-layer optical disk need to be greatly changed compared to those of a single-layer optical disk, because the effect of a front-side layer should be taken into consideration. This can complicate BCA producing equipment. In addition, when using recording materials which generally have a high wavelength dependency such as organic dyes, because the recording conditions can vary due to even a slight change in recording wavelength or in optical properties of the dye material, stable recording has been a difficult task.
Furthermore, because the recording medium of reference 1 is a rewritable phase-changeable recording medium, the recording mode during processing of the BCA is the same as the mode for deleting information in a usual recording process. In this instance, because the medium has fairly low temperature in the recording mode, it is rare that the recording layer is damaged during the processing. On the other hand, a recording mode during processing of the BCA of a write-once phase-changeable recording medium is the same as the mode for recording information in a usual recording process. In this situation, a high temperature is necessary. In addition, the recording is performed using a large laser spot, which means the recording is performed by supplying an energy that is several hundred times higher than for usual recording. Thus, a hot section is produced over a significantly large area on the recording layer, and the recording layer is damaged. This causes the problem of decreasing the signal quality when reproducing the BCA.